The present invention relates to electric motor powered vehicles and, more particularly, to a power control system and method of operation of such control system using a separately excited DC electric motor for propulsion and electrical retarding of the vehicle.
Battery powered electrically powered vehicles, such as fork-lift trucks, which generally operate at low speeds and require development of relatively large torque have historically used series wound direct current (DC) motors for propulsion since such motors exhibit the speed/torque characteristics for moving heavy materials. However, the characteristics of series wound motors are not as desirable for other types of vehicles such as golf carts or on-road vehicles where higher top end speed is desirable at the sacrifice of torque. These latter vehicles have generally adopted shunt wound or separately excited DC motors for their propulsion systems to obtain higher operating speeds. More recently, the availability of microcomputer controllers coupled with high speed, low cost switching devices have enabled the development of separately excited motor control systems which enable the shunt motor to deliver high torque at low speed and still have the high speed advantage of the shunt motor.
Whenever an electric motor is used in a battery powered vehicle, it is desirable to implement regenerative electrical retarding of the vehicle so that energy from the motor can be used to recharge the on-board batteries. With a series wound motor, electrical retarding generally only involves changing one switch to reverse the motor field. With a separately excited motor, control of field current is still required during braking with a transition to field reversal required when motor speed falls below the speed necessary to maintain regenerative current. This transition during the braking interval results in a momentary loss of braking torque which can be felt by the vehicle operator. If the system is used in a fork-lift truck carrying a heavy, unbalanced load, the momentary torque loss may cause the load to sway or fall. While one solution to this problem is to add a switch in parallel with the motor armature, this solution has the drawbacks of considerable cost, reduced reliability of the system and generation of high switching voltages which can detrimentally effect other components of the system.